The present invention relates to an apparatus and a method for grinding the periphery of an eyeglass lens to fit into an eyeglass frame.
An eyeglass lens grinding machine is known and this machine grinds a lens on the basis of the frame configuration data obtained by tracing (profiling) an eyeglass frame with a tracer. The machine has lens grinding abrasive wheels which are driven to rotate at high speed and a carriage which clamps the lens between rotating shafts and holds it rotatably. With the lens being revolved, the carriage is rotationally moved on the basis of the frame configuration data such that the distance between the axis of the lens rotating shaft and that of the abrasive wheel rotating shaft is adjusted to permit the grinding of the edge of the lens as it is brought in contact with the abrasive wheel. During the grinding operation, the carriage is rotationally moved such that the grinding pressure on the abrasive wheel is maintained constant by a spring force or the like so that no load exceeding a specified value will be exerted on the lens; hence, the lens makes a plurality of revolutions until it acquires a profile (configuration) that fits the eyeglass frame.
The conventional eyeglass lens grinding machine is designed such that the rotational speed of the lens is independent of the profile (configuration) of the lens being processed and it is held at a generally constant value. This means that in a portion of the lens to be processed to have a large size or diameter, the intended processing ends with a few number of revolutions but even after the processing of that portion has ended, the lens continues to revolve at the same speed, which eventually causes a waste of time before the processing of the whole lens is complete.
Another problem with the approach of rotating the lens at constant speed is that the point of contact between the lens and the abrasive wheel moves at different speeds depending on the shape (configuration) of the lens to be processed. Take, for example, a lens of the geometry shown in FIG. 13; areas of the lens around point A where it contacts the abrasive wheel will move very fast compared with areas around point B. This is a potential cause of introducing an error in the size or diameter of the processed lens and the error is prone to be conspicuous in a lens such as a plus lens which has its edge thickness increased toward the center.